1. Field of the Invention
This invention relates to a spectacle lens with a spherical, or rotationally symmetrical aspheric, front side and a back side (prescription surface), and also a process for its production.
Spectacle lenses are divided into those with a single refraction value and those with several, but at least two, different refraction values. The different refraction values serve to enable the wearer with limited or distorted accommodation ability to see faultlessly, at least in the near and far regions.
The different refraction values can occur in discrete steps (bifocal, trifocal) or in a smooth transition as a multifocal surface.
2. Discussion of Relevant Art
Surfaces with variable refraction value are presently preferably formed on the front side of a spectacle lens, due to current production technology and esthetics. A limited number of multiple power surfaces (multifocal surfaces) are used, since an individual tool is required for fine grinding or polishing each kind of surface to be produced. These tools have to be produced, stocked and kept ready, for each kind of surface.
In order to attain, in spite of this, the correct dioptric power (spherical, astigmatic and prismatic) at least at given points of the spectacle lens (in particular at the far and near reference points), the back surface of the spectacle lens is usually provided with a spherical or toric prescription surface that is produced with conventional machines for optical processing. A stronger toric power at the front side of a spectacle lens would be very unsightly.
Spectacle lenses are described, in present terminology, by those values for sphere, astigmatism and prism that the lens achieves at the far and near reference points. Values that deviate from these can be established at all other points of the spectacle lens, so that the dioptric power can be described by functions in x and y coordinates.
It is known that spectacle lens surfaces with multiple refraction show optimum correction results only for a design conformation. If a multiple refraction surface is used with a prescription surface that deviates from the design conformation, image quality is forfeited. In order to prevent this, a process is described, for example in German Laid-Open Patent Document DE 42 10 008 A1 (U.S. Pat. No. 5,444,503), in which an additional free form surface with small deviations from the sphere/torus is also formed on the back side.
Expanded production technological possibilities, in particular a direct processing with Fast-Tool-Servo for producing plastic spectacle lenses, make it possible to produce serviceable optical surfaces, even with strongly asymmetrical shape. No further process steps are then necessary, or only very short process steps with a flat tool (polishing). It is not necessary to use tools which are specially matched to a surface. A large number of different surfaces can thus be produced with a small number of tools by this new technique; this was previously not so.
Alternatively, processes can also be used in which a tool with a geometrically defined (milling) or geometrically undefined (grinding) cutting edge is locally in engagement with the workpiece.
A multifocal spectacle lens is described in U.S. Pat. No. 2,878,721, in which the multifocal surface is preferably present on the back side of the spectacle lens, i.e., the side facing the eye. The other surface has a spherical, toric or cylindrical shape. The multifocal surface is described in the form of regionally defined polynomials which have at least 1. deviative continual, and as far as possible also 2. deviatives p, q, r, s, t continual. In order to obtain a concave surface, as is usually required for the inner side of a spectacle lens, a spherical, toric or cylindrical surface is added to the multifocal surface, so that the power of, e.g., 6 diopters is attained in one region. The adaptation to an individual wearer is carried out explicitly with the first surface.
The addition of different surface portions is described explicitly in column 9, lines 47 ff. No suggestion is given of optimizing the surfaces for individual circumstances.
A spectacle lens with stronger refractive power and predetermined astigmatism is described in German Patent Document DE 18 05 561 (U.S. Pat. No. 3,722,986). A method is given by which astigmatism is corrected by the use of sections of ellipses. The correction strategy is expanded to segmented surfaces whose segments have different refractive powers. In the simplest case, a bifocal lens is obtained; the transition from this to a multifocal lens is effected by increasing the number of sectors.
The sectored surface is formed on the front or back side. Astigmatism can be considered at the front or back side. The sectored surface is either formed on a sphere or on a torus/cylinder. "Tuning" is referred to as the production process, starting from a sphere as a starting surface. A rotationally symmetrical aspheric body is obtained. Matching to the individual use situation is not discussed.
A spectacle lens is described in German Patent Document DE 42 10 008 A1 (U.S. Pat. No. 5,444,503) having a multi-strength surface and a free-form prescription surface.
In the Report of Guilino, Barth, New Progressive Surfaces, DOZ, November 11, 1980, page 20 ff., the construction of a multifocal surface by means of conical sections is described. It is known that the described surfaces were/are formed on the front side of the lens. The back side is configured as a sphere, or torus/cylinder, for matching to the individual case.